That by which one can pass anywhere; a safe-conduct.  Dryden.  [1913 Webster]

 A master key; a latchkey.  [1913 Webster]

 A light picture frame or mat of cardboard, wood, or the like, usually put between the picture and the glass, and sometimes serving for several pictures.  [1913 Webster]

 A strip of adhesive paper or gummed tape used to mount a picture between a piece of glass and a backing.  [WordNet 1.5]

 The method of mounting a picture between a piece of glass and a backing, using a passe-partout{4}.  [PJC]